The present invention relates to a method for producing a honeycomb sandwich panel suited for the production of a panel structure having a curved surface; particularly to a method for producing a honeycomb sandwich panel while preventing a honeycomb core from crushing, thereby simplifying the production process and thus reducing the cost and weight of the product.
Honeycomb sandwich panels constituted by a pair of prepreg skins and a honeycomb core interposed between the skins are light in weight and superior in strength and rigidity; therefore, they are used as structural members of aircraft, automobiles, etc. These honeycomb sandwich panels are generally produced by first cutting a honeycomb material into a desired shape to form a honeycomb core, covering both sides of the honeycomb core with prepreg laminate skins, mounting the resulting assembly on a forming die, if necessary, with an adhesive between the honeycomb core and the prepreg laminate, and heating the assembly under pressure for adhesion forming. In this production method, however, the edge of the honeycomb core is unable to withstand the forming pressure applied, resulting in frequent deformation (crush) of the edge. Therefore, it is desired to develop a method for producing a honeycomb sandwich panel while preventing the crush of core edge.
It is known that the crush of honeycomb core is caused by slipping of the prepregs in the skins. Japanese Patent Publication No 59-42621, for example, discloses a method for preventing the above slipping by causing projections (slipping-preventing means) of a forming die to pierce into a portion of the honeycomb sandwich panel where a lower prepreg laminate and an upper prepreg laminate are overlapping each other, inserting the projections into a press plate made of an elastic material, thereby fixing the prepreg laminates on and above the forming die. In this method using a forming die having projections, however, the projections are dangerous to an operator; the forming operation is troublesome because periodic cleaning and repair for projections are necessary; and the projections become an obstacle in sealing the forming die with a vacuum bag. Besides, there was also proposed, for example, a method for preventing the slipping of honeycomb core by fixing prepreg laminates using an adhesive tape, fixing pins, a sand paper or the like. None of these methods, however, is practical because of the problems such as increased number of steps, increased cost for forming, or the like.
A method is also known which comprises filling the edge of honeycomb core with a filler curable at room temperature, and curing the filler to increase the strength of the honeycomb core. In this method, however, widening of filling area for the prevention of crush invites increased panel weight, against the aim of lighter product weight. Moreover, because the cured filler is hard and fragile, the honeycomb sandwich panel produced may suffer cracking when the panel has a curved surface.
Further, a method was proposed which comprises providing the edge of a honeycomb core with a tapered portion to lower pressure in the forming process. The forming pressure lower than usual (about 6 kgf/cm2), however, tends to invite insufficient forming owing to pressure shortage in portions of skins where no honeycomb core is interposed. Also, an increase in the degree of tapering at the edge of honeycomb core incurs an increase in oblique dead space in a tapered portion.
Thus, the object of the present invention is to provide a method for producing a honeycomb sandwich panel suited for the production of a panel structure having a curved surface while preventing the crush of a honeycomb core, thereby achieving simplification of the production steps and reduction in product cost and weight.
As a result of intensive research in view of the above object, the inventors have found that a honeycomb sandwich panel constituted by a pair of prepreg laminate skins and a honeycomb core interposed between the skins can be produced without crushing the honeycomb core by using flat plate-shaped elastic jigs and flat plate-shaped press jigs during pressurization, or by making wider a portion of the panel where a pair of prepreg laminate skins are overlapping each other. The present invention has been completed based on this finding.
The first method for producing a honeycomb sandwich panel according to the present invention comprises the steps of (a) mounting a honeycomb core on a first prepreg laminate placed on a forming die, (b) superimposing thereon a second prepreg laminate to cover the honeycomb core to form an assembly, (c) mounting flat plate-shaped elastic jigs on a portion of the resulting assembly where the first prepreg laminate and the second prepreg laminate are overlapping each other, (d) superimposing simple flat plate-shaped press jigs independently of the shape of said forming die used on said elastic jigs so as to prevent slipping of said prepreg laminate at the following step (e), and (e) heating said assembly under a pressure of 6 to 7 kgf/cm2 for adhesion forming.
The second method for producing a honeycomb sandwich panel according to the present invention comprises the steps of (a) mounting a honeycomb core on a first prepreg laminate placed on a forming die, (b) superimposing thereon a second prepreg laminate to cover the honeycomb core to form an assembly, and (c) heating said assembly under a pressure of 6 to 7 kgf/cm2 for adhesion forming, wherein a portion of the assembly where the first prepreg laminate and the second prepreg laminate are overlapping each other has a width of 500 mm or more.
In the first method for producing a honeycomb sandwich panel, the elastic jigs are preferably made of a silicone rubber, and the press jigs are preferably made of aluminum. Further, each of the elastic jigs and the press jigs preferably has a width of 40 to 250 mm and a thickness of 0.3 to 2 mm. The forming die has a forming portion above which the honeycomb core is mounted and an edge portion above which the elastic jigs and the press jigs are mounted, an angle between the forming portion and the edge portion being preferably 30 to 180xc2x0.
In the first and second methods for producing a honeycomb sandwich panel, the adhesion forming is preferably conducted at a pressure of 6 to 7 kg/cm2. The honeycomb core preferably has a tapered portion formed by chamfering, and the edge formed by the lower and upper surfaces of the tapered portion, an angle between a lower surface and an upper surface of the tapered portion (angle of honeycomb edge) being preferably 30 to 45xc2x0.